In the days that followed the mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla., Salli Garrigan watched, riveted, in her living room while the students stood before the television cameras, sharing their anger and frustration as they demanded stricter gun laws.

Garrigan compared their boldness to her memories of the days after the shooting rampage during her junior year at Columbine High, which she had escaped by crawling across the auditorium floor and running for the door. She and her classmates had learned tricks to avoid the cameras that followed them everywhere: Stick to the middle of the crowd and never cry in public.

And now as she stood on an artificial turf field behind a microphone near McLean High School in Virginia on a bitterly cold morning, Garrigan had a message for the student protesters who had inspired her to speak.

“We left it to the grown-ups to make this right, to fix the system, and to keep us safe,” she told the teens participating in the national walkout against gun violence last week. “They told us this would never happen again.”

They had the power to fix a broken promise that was made back then, she said.

“Now as a mother and a survivor, seeing you all here today gives me tremendous hope.”

Garrigan, 35, is among a growing number of adult survivors who are joining the wave of activism inspired by the outspoken teens who survived the Feb. 14 shooting in Parkland that killed 17 students. That activism is expected to reach a fever pitch Saturday with March for Our Lives, a huge student-led rally against gun violence on Pennsylvania Avenue in Washington.

Survivors of mass shootings have increased in number over the past generation along with the pace of mass tragedy. A Washington Post analysis of shootings at schools alone has found more than 150,000 students at more than 170 campuses have experienced a shooting since Columbine.

Many past survivors are stepping into the political fray for the first time. Others are turning up the volume on earlier actions.

They are helping to fuel the growth of groups like Moms Demand Action, a grass roots gun control advocacy group founded in the days after the 2012 Sandy Hook Elementary shooting in Newtown, Conn., which left 20 children and six teachers dead.

Since Parkland, the number of new people who have participated in at least one group event per month has doubled, to 140,000. About 1.7 million new people have signed up with Moms Demand Action or its umbrella group, Everytown for Gun Safety, to receive emails or text messages or to make a donation.

The survivors are becoming an increasingly formidable political force, some analysts say.

“They speak with a different moral authority,” said Kristin A. Goss, a public policy professor at Duke University who has studied gun-related advocacy in the United States over time.

Chris Kocher, director of the Everytown Survivor Network, which also provides emotional support for survivors who are active with Moms Demand Action, credits survivors for breaking through the predicted responses from politicians after a mass tragedy.

” ‘Thoughts and prayers’ used to be an accepted response,” said Kocher, whose network of about 1,500 people is one of the largest of its kind. “But survivors have been able to say, ‘No, it’s not too soon. It’s too late’ ” to talk about gun policies, he said.

Their numbers increasingly include early survivors of school shootings who, like Garrigan, are now parents and motivated by a sense of fear and powerlessness that their children are also not safe at school.

“For survivors, sometimes the trauma is there, and it’s awakened by the experience of having children being forced to hide in bathrooms during lockdown drills,” said Moms Demand Action founder Shannon Watts.

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Mandy Collens, 33, puts a coat on her son, James, 3, before a walk. She attended her first meeting for Moms Demand Action 19 years after she ran from her high school in what was considered a “copycat” shooting after Columbine. (Sarah L. Voisin/Washington Post)

Mandy Collens, a Centreville, Va., mother of two, joined the streams of other women who signed in and searched for a seat in the cafeteria of Frost Middle School two weeks after the Parkland shooting.

She learned about the meeting from a mothers group she belongs to, but only one other woman was able to make it that night. So she sat alone in the back row.

Collens, now 33, was a freshman at Heritage High School in Conyers, Ga., when a classmate opened fire and wounded six students in 1999. The event took place one month to the day after Columbine, in what was called a “copycat” shooting at the time.

After the shooting, the adults told them they would be all right. “They didn’t know what to do,” she said.

For a long time she did not see herself as a survivor. “I may have been a couple steps from the room, but I wasn’t in the room.”

But in the 19 years since, she has had nightmares, stomach pains and panic attacks.

She found Pilates was a powerful way to clear her mind and she became an instructor. She adopted a dog. Having two children — ages 3 and 6 — has kept her busy, but it also reminds her of her worst fears.

She lingers in her car when she drops her sons off at school, she said. “I am watching — looking at everyone,” she said.

“What is difficult is never feeling like I can protect them,” she said.

Observing the Parkland students push back stirred something inside her and she decided to follow their lead.

“The difference with this movement now is that it feels like, maybe this is a way,” she said. “Maybe we are not so powerless.”

From her seat in the back of the cafeteria, Collens saw numbers flash on the screen at the front of the room. They illustrated the wide reach of gun violence: 96 Americans are killed by guns every day; two-thirds of gun deaths are because of suicide, according to the Centers for Disease Control and Prevention; 50 women each month are killed by guns in domestic violence incidents, according to the Federal Bureau of Investigation.

As organizers started talking about Parkland, she felt her heart start to quicken and her chest tighten. A volunteer came over and brought her some water, and gave her a hug.

She listened, hope rising, to Sen. Tim Kaine, D-Va., talk about new interest in introducing a measure for universal background checks in Congress this year.

Afterward, Collens went up and offered her shaking hand to the senator’s. She told him this was the first time she had come to a meeting about gun legislation.

“Those kids in Parkland gave me so much courage,” she said. “I was 15 when I ran for my life.”

After the Columbine shooting, and after the cameras went away, Garrigan sought to avoid the label of gun violence survivor. She wanted to be defined in a different way. So she pursued a career in musical theater, moving to New York City and touring with shows. Sometimes when people asked where she was from, she would offer a vague response rather than the name everyone knows for one dark reason.

She tried to hold fast to the happy memories she had from high school. But every time there was another shooting, she and her longtime best friend would check in: “How are you doing?”

Virginia Tech. Tucson. Aurora. And every year on April 20, the anniversary or Columbine.

Then four years ago, Garrigan became pregnant with her daughter, Dottie. She and her husband moved to Virginia, and she worked part-time so she could care for her daughter at home in her first years.

She was nervous when it came time for her daughter to start preschool last fall. She found a school that was nurturing and nearby, and where the front door is accessed only with a security code and guests are always escorted. “That made me feel better,” Garrigan said.

Three mornings a week now, when Garrigan gets her daughter ready for preschool, 3-year-old Dottie picks out a favorite toy or doll, and brings it with her in the car. Her mother keeps it in the morning while she is away, then brings it back when she picks her up.

“It’s a ritual, a comfort for her,” she said. It soothes her mother as well.

With her daughter in mind, last summer Garrigan attended her first Moms Demand Action meeting in Alexandria, Va. It was after the Pulse nightclub shooting, she said, and not long before Dottie would start school. “I felt more responsibility,” she said. “I thought: This is worth the fight.”

When news broke about the Parkland shooting, Dottie was napping upstairs, and Garrigan had been watching the Olympics. She was first horrified at how closely the images mirrored what she saw at Columbine. Then she was mesmerized as she watched the students respond in real time.

A few days later, she told her story to a crowd of more than 600 people at George Mason High School in Falls Church, Va., during a vigil organized by Moms Demand Action.

“How many more times do we have to live through tragedy before something changes?” she asked the crowd.

Her husband posted a recording of her speech on social media, and Garrigan said she was overwhelmed by how many high school friends responded to say, “Thank you,” and “How can I help?”

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At the rally last week, the students took turns speaking out. Some railed against members of Congress and the National Rifle Association. One cried and said she wants to be a teacher, but she does not want to have to carry a gun. Many said they are just tired of feeling scared.

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